the prototype instrument locates the sources of power

24 IEEE power & energy magazine may/june 20031540-7977/03/$17.00©2003 IEEE

The prototype instrument locates the sources of power quality disturbances and is geared toward troubleshooting and management.

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IINCREASED CONCERNS FOR POWER QUALITY (PQ) HAVE LED TO THE PROLIFERATIONof various types of PQ monitoring instruments, such as flicker meters, harmonic analyzers, and dis-turbance monitors. A common characteristic of these devices is that they are designed to monitorand characterize the power quality profiles at the metering point. They provide little informationabout the causes and directions of the recorded disturbances. As a result, these devices are usefulfor profiling the power quality conditions at the metering locations. Many experiences are neededif a user wants to use them to troubleshoot PQ problems.

PQ troubleshooting is a common task faced by utility engineers, because a very effective PQmanagement strategy is to solve the problems as they arise. The first step for PQ troubleshootingis to identify the locations of the disturbances. This requirement calls for the development of PQinstruments oriented for troubleshooting.

This article focuses on our experiences in developing a PQ instrument that is specifically gearedto power quality troubleshooting applications. The development has benefited from recentadvancements in PC-based instrumentation technology and PQ disturbance source detection meth-ods. The instrument uses National Instruments data acquisition (DAQ) hardware. This solutiontakes full advantage of a proven hardware platform without reinventing the wheel. Consequentlydevelopment efforts can be dedicated to the PQ data processing and analysis aspects of the device.This task is further simplified with the help of a graphical programming environment offered bythe Labview development system. The developed instrument is a virtual tool, a laptop equippedwith a PCMCIA card that performs all the instrumentation functions. (The PCMCIA card, alsoknown as a PC card, is a credit-card size memory or I/O device based on standards published bythe Personal Computer Memory Card International Association.) This strategy has enhanced thereliability and flexibility of the device and also reduced the development efforts.

Functional Requirements for PQ Troubleshooting InstrumentsThere are many types of disturbances that can lead to PQ concerns. It is very difficult to anticipatethe characteristics of the disturbances involved before taking measurements. Therefore, a PQ trou-bleshooting instrument cannot solely rely on the data sampling and processing schemes recom-mended by PQ standards. It must be able to acquire and present data in a way that can be furtheranalyzed by various troubleshooting algorithms. According to published PQ case studies and ourexperiences, the following functions or features appear to be very important for any PQ trou-bleshooting instrumentation:

✔ Availability of raw waveform data for further analysis: Troubleshooting PQ problemsinvolves many unknown factors. It is essential to make available the native signatures of thesystem, i.e., the raw waveform data, for further analysis. Some existing PQ monitors havelimited capability to export raw waveform data, making them unsuitable for a number oftroubleshooting applications.

✔ Flexible data acquisition schemes: The raw waveform data should be collected in formatsdesired by the user. For example, the number of cycles collected in one snapshot should beadjustable according to user’s needs. If an instrument can output only 12 cycles of wave-form, the results will not reveal interharmonic levels in the waveform. Similarly, the sam-pling rate should be adjustable to balance the conflicting needs for a long snapshot and areduced data volume.

✔ Flexible snapshots: A troubleshooting instrument should be able to take waveform snapshotsmanually and automatically in an easy-to-use manner. A trigger is normally used for auto-matic snapshots. To ensure the reliability of event capture, it is very useful to equip a trou-bleshooter with a dedicated trigger channel and with synthesized trigger signals such as thezero sequence voltages or currents.

✔ Disturbance source detection: The capability to determine the location of the PQ disturbanceis probably the most desirable feature for a troubleshooting instrument. Ideally, the instru-ment should be able to determine if a disturbance originates upstream or downstream of the

25may/june 2003 IEEE power & energy magazine

metering point. For steady-state disturbances such asvoltage imbalance and harmonics, it is highly desirableto determine the respective contributions of theupstream and downstream systems to the distortions atthe metering point since both sides could contain dis-turbance sources.

✔ Easy verification of instrument setup: Troubleshootinga PQ problem could require the execution of compli-cated and expensive field tests. It is essential to set upthe instrument correctly before the tests. A valuable fea-ture for the PQ trou-bleshooters is to havea diagnosis functionthat can check if theinstrument connec-tion and probe ratiosare established prop-erly.

On some occasions, aPQ troubleshooting instru-ment could be used forgeneral-purpose PQ moni-toring; therefore, it is alsoimportant to have somebasic PQ monitoring fea-tures built into the instru-ment. In our opinion, it isnot user friendly to have allPQ monitoring and report-ing features built into a

troubleshooter, as this may make atroubleshooter complicated to use. Thefollowing common PQ monitoring fea-tures are useful to include in the trou-bleshooter:

✔ Data logging: This feature logsthe rms and harmonic data atuser-specified intervals and forextended periods. The resultscan be further analysis to deter-mine the correlation of events todetermine if they are the cause ofincreased or decreased harmonicdistortion.

✔ Automatic disturbance capture:This feature is important for anyPQ instrument. Some PQ moni-tors have the capability to dis-play the severity of a disturbanceon the PQ envelope. For PQtroubleshooting applications, itis more useful to have the rawwaveform data associated withthe disturbances saved for fur-ther analysis.

Like many common PQ monitors, a PQ troubleshootinginstrument must have two operation modes. One is the real-time mode in which the data are collected and classified. Theother is the post-processing mode in which the data are furtheranalyzed to determine the source and severity of the distur-bances. A functional chart of the PQ troubleshooter is shownin Figure 1. The blocks above the Internal Data Files bar inFigure 1 are the real-time part, operating in real-time at themetering location. Usually this part cannot afford heavy com-putation due to the requirement for real-time data collection.

IEEE power & energy magazine may/june 2003

figure 2. Manual snapshot for flicker troubleshooting.

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figure 1. Desirable features for PQ troubleshooting applications.

Real-Time Data

WaveformSnapshot

DisturbanceCapturing

DataLogging

StatisitcalAnalysis

CorrelationAnalysis

Internal Data Files

Harmonic,Interharmonic

and FlickerAnalysis

SourceDetection

HarmonicContributionSeparation

Sag, Swell,and Transients

Analysis

DisturbanceSource

Detection

Troubleshooting Report: Data, Charts, Tables, and HTML Files

may/june 2003 IEEE power & energy magazine

Further analysis is done on the collected data with the post-processing components of the troubleshooter, which areshown below the Internal Data Files bar. In addition to dataanalysis, this component should also be able to output theresults in charts and tables suitable for inclusion in a trou-bleshooting report.

Instrument Design and Application ExamplesThe following application examples demonstrate some of thefunctional requirements discussed earlier and the possibleways to implement these functions.

Waveform SnapshotsThe purpose of waveform snapshots is to provide users suffi-cient raw data for further analysis. This feature is extremelyimportant for PQ troubleshooting. Some existing PQ monitorsreport processed data only. A disadvantage of using processeddata is that the true conditions experienced by the system maynot be revealed, since the data have been manipulated by aseries of built-in algorithms. The algorithm designers usuallycannot anticipate all the needs and problems faced by users.Some PQ meters can report limited raw data. A common incon-venience when using these meters is that users are only allowedto take snapshots with an instrument-specified window size,such as 12 cycles. Snapshots with short window size cannotprovide sufficient information for PQ troubleshooting, especial-ly for slow-evolving disturbances, such as trans-former energization or light flicker.

There are two snapshot modes required forPQ troubleshooting: manually triggered andautomatically triggered snapshots. Manualsnapshot enables users to record signal wave-forms at their discretion and is very useful forsteady-state PQ problem troubleshooting. Animportant requirement for manual snapshot isthat the window size should be configurable.This is essential for interharmonic or flicker-related troubleshooting, because Fourier analysis requires along window size to detect the nonintegral order harmonics.

As an example of manual snapshot application, a prototypePQ troubleshooting instrument was used to monitor the PQ ofan industrial plant with a large variable frequency drive (VFD)load. The plant and nearby customers have been experiencinglight flicker for some time. The objective of the test was todetermine the cause of the flicker. In the test, several manual

snapshots were taken, and one sample is shown in Figure 2.The top graph in Figure 2 plots the phase A current waveform,where obvious ripples can be observed in the envelope. Thebottom graphs plot the voltage and current spectra of therecorded waveforms. A 60-cycle window size is used in thisanalysis, since it can yield spectrum resolution of 1 Hz. Thespectra show a significant component at 236 Hz, which isclose to the fourth harmonic. To confirm that this nonintegralorder component is genuine interharmonic instead of spec-trum leakage, one can examine the correlation between volt-age and current of the component. A simple method forcorrelation analysis is to apply a sliding window along thewaveforms to check the consistency between the voltage andcurrent trends. Figure 3 shows the voltage/current magnitudetrends for the 236-Hz component when a 60-cycle slidingwindow is used. The waveforms indicate that the voltage andcurrent have good correlation, which confirms the existence of236-Hz interharmonics. Further source detection analysisrevealed that the drive is the source of the interharmonics. Forthis example, if a long snapshot window were not available,the detection of interharmonics, the source of the light flicker,would be very difficult.

Another mode of snapshot function, automatic snapshot, isapplied to record waveforms under user specified conditions.The specified condition can be a predetermined time or athreshold (also called “trigger”). This feature is important foradvanced PQ troubleshooting. For example, it can be used to

capture events related to transformer energization (inrush cur-rents) or capacitor switching. In addition to the flexible win-dow sizes required in manual snapshot, automatic snapshotshould provide flexible trigger parameter setup. An importantparameter is the signal selected for triggering. An ideal trigger-ing signal must be sensitive to the event of interest but insensi-tive to variations of normal operating conditions. Our field testexperiences show that neutral current is a very effective trigger

figure 3. Current (left) and voltage (right) trends of 236 Hz component.

Use of a laptop equipped with a PCMCIA card that performs all the instrumentation functions enhanced the reliability and theflexibility of the device and reduced development efforts.

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signal for capturing switching disturbances. As a result, wehave built a synthetic neutral current that is the summation ofthe three phase currents as a trigger option in a prototype PQtroubleshooter. This approach saves the need to wire and meas-ure the zero sequence current for the instrument.

Data Trending AnalysisThe data trend analysis feature can contribute to PQ trou-bleshooting in two aspects. One is to provide a statistical sum-mary of some key PQ indices so that users can have an idea ofhow troublesome the system condition is. A widely used toolfor such statistical study is an occurrence histogram plot.Another aspect, which could be more important, is to correlatethe trends among themselves or with known system events.

This is useful, for example, todetermine if an increased har-monic distortion is due to theenergization of a shuntcapacitor.

An example of trendanalysis is shown in Figure 4,where a 25-kV feeder wascontinuously monitored forabout 16 hours with a logginginterval of one minute. Thefigure displays voltage THDtrend (upper graph) and itshistogram (lower graph).From the histogram, we canfind that the voltage THD isbelow 2.38% for 95% of thetime. The monitoring periodincludes the switching in and

switching out of a capacitor bank. The THD step changesshown in the trend graph are the results of these switchingevents. The effects of the capacitor on harmonic distortion canbe further assessed by correlating the responses of key har-monic components to the switching events. Figure 5 correlatesthe fifth and seventh harmonic voltage magnitudes during amonitoring period. It can be seen that both components weresensitive to the switching operations, but they change in oppo-site directions. When the capacitor was switched in at 16:12and 16:38 on 23 October 2002, the fifth harmonic voltageincreased by approximately 100 V, but the seventh harmonicdecreased by 20 V. When the capacitor was taken out at 16:24,the fifth harmonic dropped, but the seventh rose correspond-ingly. From this correlation check, we could draw a prelimi-

nary conclusion that the system appears resonant at thefifth harmonic when the capacitor is in operation, andthe capacitor serves as a sink for the seventh harmonic.

Automatic Disturbance CaptureThis feature is important for any power quality instru-ment. It can tell what kinds of disturbances are experi-enced by a system and how often they occur. PQinstrument manufacturers may use different distur-bance capture methods and report the event with dif-ferent formats. Accordingly, the usability of theirresults for PQ troubleshooting might differ greatly.

For disturbance capturing, many traditional PQmeters require users to set a threshold for a specificvariable. The variable is then scanned to find themoment when its value exceeds the threshold. Thistechnique is similar to the aforementioned automaticsnapshot. We find, however, that it has some draw-backs for PQ troubleshooting applications. The firstdrawback is that a user must have good knowledgeabout the typical range of the trigger variable in orderto set a proper threshold. The second drawback is that


figure 5. The fifth (top) and seventh (bottom) voltage harmonicmagnitudes correlation analysis.

figure 6. Automatic capturing of a capacitor-switching event.

figure 4. Data trend analysis.


the instant of event occurrence may not be accurately locat-ed if the threshold is set inappropriately. For example, if thethreshold is set too high, the disturbance may be capturedafter several cycles of the actual event occurrence. If it is settoo low, normal noise or load change may be wrongly cap-tured as disturbances. To overcome these disadvantages, weuse a method that compares the voltage or current wave-forms of two consecutive cycles. If the waveform differenceexceeds a threshold, a disturbance is assumed to haveoccurred, and the instant of occurrence is determined. Thismethod is based on the observation that the voltage or cur-rent waveform variation between two consecutive cycles isusually very small for any steady-state operating conditions.The method has been proven to be very robust according tofield tests on systems ranging from 120 V to 500 kV. Thereis no need to change the built-in threshold unless the signalis extremely distorted or noisy. As an example, Figure 6shows a capacitor-switching event. The voltage and currentwaveform change indices used for this capturing are shownin Figure 7. It can be seen that the waveform changed sig-nificantly at the instant of switching. Before and after theevent, the waveforms are stationary.

When reporting captured events, some existing instru-ments provide a severity report or other summary informationonly. For example, a sag disturbance is reported as a dot on thePQ envelope. This reporting feature is generally sufficient ifone considers only the PQ status of the monitored system. ForPQ troubleshooting, raw waveform data is more useful andshould be made available for further analysis. One exampleapplication is to load the raw data into a post-processing toolfor disturbance source detection.

Disturbance-Source DetectionThis is probably one of the most desirable features for PQtroubleshooting devices. With this feature, one can find fromwhich direction a PQ disturbance originates. Thedevice can then be moved to a new monitoring pointestablished by the directional information. The ori-gin of the disturbance could eventually be trackeddown in this manner. For steady-state disturbancessuch as harmonics, it is also desirable to determinethe relative contributions of the upstream and down-stream systems to the harmonic distortion level atthe monitoring point. The University of Alberta hasbeen investigating techniques for disturbance-sourcedetection for some time. By combining the advan-

tages of various published methods, we managed to imple-ment some practical source-detection techniques into a proto-type PQ troubleshooting instrument. The following harmonicfield test example demonstrates how this feature functions.

The test system is a 25-kV feeder of a 138-kV substation.The current and voltage waveforms at the metering point areshown in Figure 8. The fifth harmonic was found to be thedominant component. The requirement for PQ troubleshoot-ing in this example is to determine the relative contributions ofthe harmonic sources in the upstream and downstream sys-tems to the fifth harmonic distortion recorded at the meteringpoint. The top graph in Figure 9 shows the measured magni-tudes of the harmonic current sources in the upstream anddownstream systems. The bottom graph shows each source’sactual scalar projection contribution to the voltage distortionmeasured at the metering point. From the top chart, one cansee that the upstream side has larger harmonic current injec-tion. The bottom chart shows that the upstream source con-tributes almost all of the fifth harmonic voltage distortion atthe metering point.

Verification of Instrument SetupPQ troubleshooting could involve expensive field tests. Some

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Relying on data sampling/processing schemes recommended by PQ standards, the instrument acquires and presents data forfurther analysis by troubleshooting algorithms.

figure 7.Waveform

changeindices:

upper,current;

lower,voltage.

figure 8. Voltage and current waveforms of a 25 kV feeder.

intrusive tests cannot be repeated due to system restrictions. Itis, therefore, necessary to set up the instrument correctly beforethe tests. This requires the PQ troubleshooting instrument tohave a self-diagnosis feature for verification of instrument con-nection and probe setup. The purpose of the self-verification isto check if the magnitude, polarity, and phase sequence of theinput signals are consistent with user’s configuration. Since thesignal transducer configuration can differ from system to sys-tem, the self-verification feature must adapt to it automatically.For example, one system may have full three-phase currentsand voltages accessible, while another system may have two-and-a-half element meter connections only. Another importantfeature for performance verification is to display the measuredwaveform or summary data in real-time so that the user canknow if the instrument is running properly. Since the PQinstrument is often left in the field for long-term data collect-ing, this feature is especially useful when a user needs to checkthe instrument operating status.

In our prototype troubleshooter, the instrument setup is

verified by displaying line power flow, phase sequence, degreeof voltage imbalance, and a few other parameters. This isachieved by allowing the user to click a verification button.The diagnosis results are then displayed for the user to verify.Recommendations are also given as to how to reconnect thechannels if the results don’t make sense. A real-time, scope-like, harmonic-distortion display is also available for a user tocheck the status of the system.

Virtual InstrumentThere are a number of ways to construct a PQ troubleshooterdiscussed in the previous sections. Our survey indicates thatthese methods can be broadly classified into three groups.

✔ Use of digital signal processors or similar hardware: Alarge majority of existing PQ monitors are constructedwith this approach. The major advantage of thisapproach is that the developer is free to customize anyfeatures to his or her liking, since there is almost unlim-ited control over the system. The main disadvantage ofthis approach is that the development begins from thevery basic hardware level. A lot of learning and devel-opment efforts have to be made on areas already mas-tered by digital hardware professionals.

✔ Combined use of custom hardware and a commercialDAQ card: Some PQ monitors utilize a commercialDAQ card but customize their own signal-conditioningcircuits. This approach saves the DAQ circuit-designeffort, but it requires the design of a special peripheralcircuit for the DAQ card. A major disadvantage of thisapproach is that the developer must grasp the detailedbehavior of the DAQ card very well. Once the commer-cial card is upgraded, the custom hardware may requireredesign. For both approaches, the developer must writesoftware based on general programming languages,figure 10. Virtual power quality instrument.


figure 9.Separation of fifth

harmonic contribu-tions from upstream

and downstream.


such as Visual C++ or Visual Basic. The programmingeffort can be huge, because these languages are notgeared to data acquisition and analysis applications.

✔ Use of a commercial DAQ hardware and softwaredevelopment platform: This approach takes full advan-tage of the commercial signal-conditioning and DAQhardware. It significantly reduces hardware develop-ment effort. With the companion software-developmentplatform that is specially designed for measurementapplications, the software programming effort can bereduced significantly. The disadvantage of thisapproach is that the developer has to comply with theprovided hardware/software structure and rely on sup-port from the vendors.

We believe that the use of a commercial DAQ hardware andsoftware development platform is likely to be the future direc-tion for developing advanced PQ instruments. Several yearsago, this approach was not available or the DAQ hardware wasnot powerful enough. However, it has become a very attractiveapproach in recent years for many engineering disciplines.

One of the proven technologies for this solution is theNational Instruments DAQ hardware and Labview develop-ment system. Supported by this hardware and software envi-ronment, more development efforts can be dedicated to PQprocessing and analysis related subjects, an area in whichpower engineers have their strength. The instrument devel-oped is a virtual instrument, since a laptop equipped with aPCMCIA card performs all instrumentation functions. Thevirtual PQ troubleshooting instrument discussed in early sec-tions is shown in Figure 10.

Future DevelopmentsThere is an increasing need to locate the sources of powerquality disturbances for improved PQ troubleshooting andmanagement. The PQ troubleshooter featured in this articlegives an example of how to satisfy this need. Although theprototype instrument has demonstrated disturbance sourcedetection and other power quality diagnosis techniques, fur-ther research effort is needed to improve the techniques.

In the next step, we plan to test the instrument in otherCanadian utility systems. Some of the tests are essentially totroubleshoot real-life PQ problems. We welcome other inter-ested users to try the instrument.

Our experiences with the prototype device indicate that thevirtual-instrument approach has the advantages of proven reli-ability and reduced development efforts. The real-life exam-ples demonstrated in this article have shown the promisingperformance of virtual instruments for PQ troubleshootingapplications.

AcknowledgmentsThe authors acknowledge the financial and field test supportof the Natural Science and Engineering Research Council ofCanada, Alberta Science & Technology Authority, and B.C.Hydro.

Further ReadingA. McEachern, W.M. Grady, W.A. Moncrief, G.T. Heydt, andM. McGranagham, “Revenue and harmonics: a evaluation ofsome proposed rate structures,” IEEE Trans. Power Delivery,vol. 10, no. 1, pp. 474-482, Jan. 1995.

A.C. Parsons, W.M. Grady, E.J. Powers, and J.C. Soward,“A direction finder for power quality disturbances based upondisturbance power and energy,” IEEE Trans. Power Delivery,vol. 15, no. 3, pp.1081-1086, July 2000.

H. Yang, P. Porotte, and A. Robert. “Assessing the har-monic emission level from one particular customer,” in Proc.3rd Int. Conf. Power Quality: End-Use Applications and Per-spectives, Amsterdam, The Netherlands, 1994.

W. Xu and Y. Liu, “A method for determining customerand utility harmonic contribution at the point of common cou-pling,” IEEE Trans. Power Delivery, vol. 15, no. 2, pp. 804-811, Apr. 2000.

C. Li, W. Xu, and T. Tayjansanant, “A critical impedancebased method for identifying harmonic sources,” IEEE Trans.Power Delivery, to be published.

BiographiesChun Li is a postdoctoral fellow at the Electrical and Com-puter Engineering Department of the University of Alberta.He received his B.S. and Ph.D. degrees from Tsinghua Uni-versity, China. His interests are harmonics and power quality.He may be reached at [email protected].

Wilsun Xu is a professor at the University of Alberta. Heworked for BC Hydro from 1990 to 1996 as an engineer. Hereceived his Ph.D. degree from the University of BritishColumbia. His main research interests are power quality andharmonics. He may be reached at [email protected].

Brent Hughes is manager of Revenue Metering and Dis-tribution Standards at BC Hydro. He received his B.S. andM.S. degrees in applied science in electrical engineering fromthe University of British Columbia. He is a member of IEEE,the Canadian representative to Cigré Study Committee 36dealing with electromagnetic compatibility, and a registeredprofessional engineer in British Columbia, Canada.

James Gurney manages BC Hydro’s Strategic Researchand Development Program. He received his B.S. degree inapplied science in electrical engineering from the University ofBritish Columbia. He is a registered professional engineer inBritish Columbia and an IEEE Senior Member. He has partic-ipated in the development of a number of IEEE standards andhas served for several years on the IEEE Standards AssociationStandards Board, most recently as vice chair.

Bruce Neilson is a senior engineer in electrical technolo-gies at Powertech Labs. After completing a Ph.D. degree inplasma physics, he joined BC Hydro R&D, now PowertechLabs. His main effort recently has been in developing digitaltest and measurement techniques that have been applied topower quality studies, field measurements, and laboratorytesting. He is chair of the Canadian Standards Association(CSA) subcommittee on uninterruptible power supplies.

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the prototype instrument locates the sources of power

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